Conventionally, the exhaust gas from internal combustion engines, such as diesel engines, contains nitrogen oxides (hereinafter referred to as “NOX”), which can cause environmental pollutions. An SCR (Selective Catalytic Reduction) system using a urea solution as the reducing agent is known as an exhaust gas purification apparatus for reducing the NOX (NO and NO2) to purify the exhaust gas.
This SCR system is as follows. The urea solution as the reducing agent is supplied into an exhaust gas passage at an upstream side of the reduction catalyst. The ammonia produced by hydrolysis of the urea solution is adsorbed by a reduction catalyst. NOx that flows into the reduction catalyst is caused to react with the ammonia, so that the NOx is decomposed into nitrogen, water, carbon dioxide, and the like and then emitted.
A known method for supplying the reducing agent in such an SCR system is as follows. For example, a urea solution stored in a storage tank is pumped by a reducing agent supply pump, and the urea solution is supplied into an exhaust gas passage by a reducing agent injection valve disposed so that an injection hole thereof faces the exhaust gas passage.
In the case where a urea solution is used as the reducing agent, when the amount of the reducing agent injected is too small relative to the amount of NOx contained in the exhaust gas, the NOx reduction efficiency becomes poor. As a consequence, there is a risk that the NOx that is not reduced may be emitted. On the other hand, when the amount of the reducing agent injected is too large, the ammonia produced by hydrolysis of the urea solution may slip to a NOx downstream side. Ammonia shows relatively high toxicity. For this reason, the ammonia is oxidized and reduced to NOx that has relatively low toxicity by placing an oxidation catalyst in the downstream side of the reduction catalyst, or an ammonia decomposing catalyst is provided to decompose the ammonia into nitrogen and water, so that emission of the slipped ammonia into atmosphere can be prevented. In other words, when the amount of reducing agent injected is too large, there is a risk that NOx may be emitted in a similar way to the case where the injection amount is too small. Moreover, the consumption efficiency of the reducing agent reduces.
In view of this, a control method for an exhaust gas purification apparatus that optimizes the supply amount of reducing agent has been disclosed. More specifically, the method disclosed is a control method for an exhaust gas purification apparatus that reduces and purifies NOx in the exhaust gas by adding a reducing agent to the upstream side of a NOx reduction catalyst incorporated in an engine exhaust gas passage. The method is characterized as follows. A target adsorption amount curve in which a saturated adsorption amount curve representing the relationship between the catalyst temperature and the saturated adsorption amount of the reducing agent to the NOx reduction catalyst is shifted to a low temperature side is set. The target adsorption amount of the reducing agent corresponding to the catalyst temperature is calculated, and the actual adsorption amount of the reducing agent to the NOx reduction catalyst is obtained. The addition amount of the reducing agent to the upstream side of the NOx reduction catalyst is reduced when the actual adsorption amount reaches the target adsorption amount, while the addition amount of the reducing agent is increased when the actual adsorption amount becomes lower than the target adsorption amount. In addition, a control method in which a predicted temperature of the catalyst after a predetermined time is obtained and the amount of the shift of the target adsorption amount curve is corrected according to the predicted temperature of the catalyst has been disclosed as one embodiment of the control method. (See Patent Document 1.)
Patent Document 1: JP-A-2006-22729 (the entire text, all the drawings)